Project Summary: EGR1 (Early Growth Response 1) is a well-known transcription factor of the immediate early gene family. My lab recently discovered an unexpected and intriguing role for EGR1 in regulating the activity of blood- forming hematopoietic stem cells (HSC). In particular, our extensive Preliminary Data demonstrate that EGR1 normally functions both to limit HSC proliferation and to promote HSC retention in the BM niche. These findings are significant for several reasons. First, EGR1 represents one of only a few known regulators of HSC quiescence, which is critical in preserving HSC function. Moreover, EGR1 represents the first identified transcriptional regulator of HSC migration, suggesting that targeting this factor may provide novel avenues to inducing stem cell mobilization for clinical transplant. Finally, and most remarkably, the role of this single gene in determining both the proliferation and anatomical localization of HSC reveals a novel and potentially broadly acting mechanism for controlling stem cell number whereby stem cell division is molecularly coordinated with retention in the niche. Egr1-/- mice thus present an extraordinary opportunity for discovering new and important insights into the fundamental properties of HSC and their clinical applications. Through two focused and complementary Specific Aims, the work proposed in this application will (1) determine by functional analyses whether the stem cell regulatory factor Bmi1 represents a key target gene responsible for EGR1-mediated effects on HSC proliferation and localization, (2) identify and functionally validate additional, novel targets of EGR1 that regulate HSC activity, and (3) elucidate the cellular mechanism(s) underlying the spontaneous mobilization of LT-HSC in Egr1-/- mice. These studies thus take advantage of EGR1 and Egr1-/- mice as unique model system to answer long-standing questions about the molecular and cellular regulation of HSC function.